For measuring a thickness of a film formed on a substrate, there exist an ellipsometer (which is disclosed in Japanese Patent Application Laid Open No. 2009-68937, for example) and a measuring device in which a film thickness is obtained from the wavelength which shows maximum or minimum in spectral reflectivity (which is hereinafter referred to as a PV (Peak-Valley) device and disclosed in Japanese Patent No. 3532165, for example).
The ellipsometer is widely used for measurement of a thin film thickness in the field of manufacturing semiconductors. However, the ellipsometer has the following problems. Since the projection angle and the receiving angle are large, the ellipsometer can hardly be used in a process in which a distance to the object will change. Since optical elements both on the projection and receiving sides have to be rotated for the measurement, the optical system is complicated and expensive.
Since, in the PV device, a film thickness is obtained from the wavelength which shows maximum or minimum in spectral reflectivity, it is necessary that the wavelength which shows maximum or minimum in spectral reflectivity should exist. However, in general, in spectral reflectivity data of thin films thickness of which is 500 nm or less, the wavelength which clearly shows maximum or minimum will not exist. Accordingly, the conventional PV device cannot be used for measurement of thin films thickness of which is 500 nm or less.